The present invention relates to the field of gels. The invention also relates to an improved method for manufacturing gels.
Gels are versatile soft solids (that is, materials that are between solids and liquids) with useful properties. They are used in many products in a wide range of applications, for example; personal care, foodstuffs, drilling muds and pharmacological uses. Many of these properties exploit the presence of a finite elastic modulus, accompanied by yield behaviour enabling flow under large stresses. Various routes to the creation of gels exist, but since the variety of product behaviours required is almost limitless, any new generic technology for gel creation opens wide possibilities for new and improved products.
Some gels consist of equilibrium structures in a given range of temperatures for a given state of chemical bonding. However, other gels are non-equilibrium arrested states whose properties depend on process history. The properties of non-equilibrium gels can be tuned during formulation and manufacture, making them useful in product design.
Fluid-bicontinuous gels are known to the art. In this context, “fluid-bicontinuous” denotes that at any instant, two continuous inter-penetrating domains, each containing a different fluid, are present throughout a gel. Particle-stabilised materials, including particle-stabilized gels, are also known to the art. The two fluids are inter-meshed such that the surfaces of the fluids are in contact with each other. However, both these gels and materials have limitations and shortcomings which restrict the extent to which their physical and chemical properties can be controlled to meet desired criteria.
Also, fluid-bicontinuous gels have different physical properties from gels that comprise discrete droplets. For example, discrete droplets form a fluid phase at low density but transform into gels under compression by either external or internal (bonding) forces; this allows gels to be created from traditional emulsions stabilized by surfactant molecules. The differences in physical properties can also include enhanced stability under gravity and/or under exposure to solvents. In particular, droplet emulsion gels may dissolve if exposed to an excess of their continuous phase. Moreover, the property of fluid bicontinuity is itself desirable in certain applications, such as a gel through which hydrophobic and hydrophilic molecules are both freely permeable. However, as will become apparent from the discussion of the prior art below, known fluid-bicontinuous gels are generally equilibrium structures that cease to be stable upon temperature change and/or exposure to external solvents.
Fluid-bicontinuous states of two solvents can be created temporarily. These temporary states may be established in several ways. One such method is to apply a high level of agitation to a system where two solvents have roughly equal volume fraction and viscosity. Another is to raise the temperature of a mixture of two immiscible solvents until the fluids become miscible, allow them to mix microscopically, and then quench the mixture to below the critical temperature, at which point the fluids become immiscible, so that spinodal decomposition (transition from a single phase into two separate phases) occurs. This is well known to give bicontinuous structures (for solvents of similar viscosity), when the fluid:fluid composition ratio is in the range 30:70 to 70:30, with the most robust examples being around 50:50. However, without stabilisation of some sort, such bicontinuous states are transient.
Fluid-bicontinuity can be temporarily sustained in a system where one or more solvent is a high molecular weight polymer of high viscosity. However, to trap the structure it is necessary to pass below the glass transition temperature of the relevant polymer so that it becomes a solid phase. As such, the resulting structure is no longer fluid-bicontinuous. Impermanent fluid-bicontinuous systems of this type are temporarily sustainable and only at restricted temperatures. Also, they may be denatured by the presence of external solvents.
A fluid-bicontinuous state can also be obtained by the addition of a surfactant which absorbs onto the interface between two fluids. The bicontinuous state is itself generally highly fluid, thermodynamically stable and the resultant material is commonly called a bicontinuous microemulsion. This is not a gel. Fluid-bicontinuous gels can however be obtained at high enough surfactant concentration, in the form of bicontinuous (cubic) liquid crystals. These are thermodynamic phases, stable within a modest temperature range only. Their pore-size and elastic properties can only be varied within a very limited range.
Surfactants and their mixtures are also widely used to stabilise ordinary emulsions (as opposed to microemulsions), which are thermodynamically metastable. Such metastable emulsions are generally not bicontinuous but are a dispersion of spheres of one fluid in the other. These emulsions remain liquid unless the dispersed phase has a high volume fraction, in which case a biliquid foam, which is a type of gel, is produced. However, such gels comprise discrete emulsion droplets and therefore are not fluid-bicontinuous. In addition, biliquid foam gels are stabilised by non-rigid surfactant monolayers, and thus are not particle-stabilised. Furthermore, they can be denatured by coming into contact with external solvents.
Certain structures, often called Pickering emulsions, utilise near-neutral wetting (NNW) particles to stabilise discrete, usually spherical emulsion droplets. In this context, near-neutral wetting particles are particles that span the interface between two immiscible fluids, such that the angle at the fluid-fluid-solid contact line is not too far from 90 degrees. NNW particles are a subset of partially wetting (PW) particles—particles that have a contact angle that is strictly greater than 0 degrees and less than 180 degrees. The angle 90 degrees is known as the neutral wetting (NW) angle. The NW angle of 90 degrees is included when referring to NNW particles, the NNW angle and PW particles. NNW particles are known to have a strong affinity to the interface between fluids. Once contacted by such an interface they are attached almost irreversibly. In particular the timescale for such particles to become detached from the interface by Brownian motion is extremely large. As Pickering emulsions are comprised of discrete emulsion droplets, stabilised by NNW particles, they are not fluid-bicontinuous, nor usually are they gels.
However, if the particle coverage on the fluid-fluid interfaces within a Pickering emulsion is sufficient, these interfaces are known to become locally rigid, even if the interaction between the colloidal particles is repulsive. This is because these particles are jammed together by the tendency of a fluid-fluid interface to reduce its area. Such rigidity does not in general impart macroscopic rigidity to the sample, because a suspension of droplets with rigid surfaces is not in general rigid. Note that a rigid interface can be considered as an interface substantially covered in particles, the particles being forced into intimate contact such that they have restricted movement, thereby imparting a substantial amount of inflexibility to the interface.
Gels formed by compression of Pickering emulsion droplets are known, which again are stabilised by particles but are not fluid-bicontinuous (see EP 0309054, U.S. Pat. No. 2,968,066 and Materials based on solid-stabilized emulsions, F. Leal-Calderon et al., Journal of Colloid and Interfacial Science, 275, 2004, 659). These stabilised compositions become macroscopically rigid only when the droplets are pressed into contact. That is, they generally require an external force acting on them in order to establish macroscopic rigidity (although in some cases the drainage force of gravity, or internal attractions among the droplets, will suffice). Particle-stabilised gels created by compression of Pickering emulsions generally comprise droplets, and are not fluid-bicontinuous. Furthermore, they can be denatured by coming into contact with external solvents. In particular, they can be dissolved in a solvent comprising the same fluid as the continuous fluid phase of the emulsion, or another fluid miscible with that fluid.
In summary therefore, the prior art materials referred to above have significant limitations in terms of their properties, their function and their tunability. Some prior art materials provide temporary macroscopically rigid structures, but these are influenced by external forces (or the lack thereof), such as drainage under gravity. Most of the prior art materials are affected by the presence of external solvents. Many of the materials exist as thermodynamic equilibrium states and therefore cannot be tuned during processing, and cannot be maintained as gels outside the narrow range of thermodynamic conditions for which they are in the equilibrium state. Others have only short-lived existence.